The invention concerns a light-optical detection device for determining the concentration of suspended particles in a gaseous medium within a light-tight housing. The latter essentially is formed by a short cylinder sheath having two front-surface coverings that are formed by a base plate and by an opposite cover plate. Cylinders extend into said housing, for receiving an optical detection system with a light source and a photocell.
Such a device is known from the West German Auslegeschrift No. 1 598 941. In this device, the cylinder with the photocell is arranged in the base plate and the cylinder with the light source is arranged in the cyinder sheath. In addition, a light trap is provided opposite the light source, and this light trap likewise extends as a cylinder outwardly away from the cylinder sheath. Because of this arrangement of the individual parts of the optical detection system, the structure has considerable spatial extension. Furthermore, it should be noted that the cylinders with the light source and the light trap cannot be moved relative to one another, since this would significantly impair the optical action of the light trap. If the light trap is to be optimally effective, both cylinders must be aligned precisely axial with respect to one another. Furthermore, with the known device, it is necessary that the base plate maintain an adequate distance from the mounting plane, which is generally formed by the ceiling of a room that is being monitored.